This invention relates to optical fibers and optical fiber components. It relates in particular to an all-fiber depolarizer enabling the change of an optical signal polarization state, from a strongly polarized state to an unpolarized or depolarized state.
Laser light transmitted through optical fibers is usually a polarized light because lasers and laser diodes are strongly polarized light sources. There are however some applications where the polarization of the light is not desirable. In test systems, the results are not influenced by polarization dependent loss (PDL) if the light source is unpolarized. In Raman amplification systems, the Raman effect is polarization dependent, thus the need for an unpolarized source for higher uniformity performance.
There are only a few truly unpolarized light sources available. An incandescent lamp or an arc lamp will produce unpolarized light, i.e. light that contains all states of polarization at the same time. However, these sources are big, produce a lot of heat and thus have low energy efficiencies. It is very inefficient to couple them to single-mode optical fibers. To build a bulk laser that is unpolarized, one can build a laser cavity that has no polarization-dependent element. This is possible with a gas laser that has dielectric mirrors which are not placed at angles with the axis of the cavity. Though these lasers can be coupled relatively efficiently to optical fibers through lenses, they are also bulky and not very energy efficient. To obtain higher power efficiency, one must use a semiconductor diode laser, either as a pump for fiber lasers or as the laser source itself Because of their structure, laser diodes are highly polarized light sources. Only a vertical cavity diode can be unpolarized, but it can not have a high power emission because of the extreme size length of the laser cavity. Thus such laser diodes cannot be used as pump lasers in Raman amplifiers.
With a non-birefringent rare-earth doped fiber, such as erbium doped fiber, one can build unpolarized light sources, such as a broadband amplified spectral emission (ASE) source or even a fiber laser, because they are made with non-polarization selective elements such as broadband WDM fused fiber couplers. These sources are powerful and easy to couple with fibers, but are much bulkier than semiconductor diode lasers. There is thus an interest to depolarize sources such as semiconductor diode lasers.
The purpose of the depolarizer is to transform the output of a polarized source, which has a high degree of polarization (DOP), typically close to 99%, and to reduce that degree of polarization to a very small value, typically a few percent. One cannot truly depolarize a polarized light source, but one can simulate an unpolarized source by shifting the state-of-polarization so fast that the system or the detector averages all the states of polarization. This can be done in time and/or in wavelength. To realize a depolarizer strictly in time, one can actively modulate the polarization. This is usually done with piezoelectric elements that modulate lengths of a birefringent fiber, thus rotating the state of polarization as fast as the piezoelectric element can be modulated. This method works, but does not insure that all-states of polarization are always on average passed through. It will depend on the speed of the detector or the system. This is also an active device, which is useful in test systems, for example because it is more or less universal. In systems or sub-systems however, such as Raman amplifiers, one prefers a passive depolarizer rather than an active one.
Passive depolarizers have been produced using fiber delay lines. The basic principle is to mix different states of polarization, delayed in time, through a fiber loop or fiber lengths. The principle behind passive depolarizers makes them sensitive to the bandwidth of the laser. A very broad source (tens of nanometers) is easy to depolarize, whereas very narrow band lasers, such as a single-mode laser, are almost impossible to depolarize completely because their coherence lengths are very long.
U.S. Pat. Nos. 5,933,555 and 6,205,262 describe a depolarizer based on recirculating loops. To achieve a depolarized state, a polarized light source is split through a 2xc3x972 (2 inputs, 2 outputs) splitter, with a given amount of power being split in the recirculating loop. The rest of the power goes to the transmission fiber. The splitter""s optical function is to split light from either of the input ports of the splitter to the two output ports, at a given ratio, i.e., 50%/50%, 33%/67%, etc. This split light goes through the loop back to the second input port of the splitter and thus is partly recombined in the initial transmission fiber. Because there is no control over the polarization in the loop, the amount recombined is in a random orientation with the initial input light. The rest goes back into the loop and again is recombined at a random state, etc. until the amount recombined is negligible. Though this principle does depolarize the light, it only performs a partial depolarization. The depolarization efficiency will depend on the coupling ratio and the polarization changes in the loop. To obtain a relatively uniform depolarization as a function of polarization input states and wavelength, one must use a broadband splitter and cascade many such recirculating loop depolarizers so that if the first loop reduces the degree of polarization to 40%, the second will reduce it to (0.4)2=16%. To obtain a very low DOP up to 15 such stages are required. One can make the device a bit more efficient by placing a polarization controller between the loops and either use birefringent fiber or place a further polarization controller in the loop, but this still produces a very bulky arrangement to obtain desired depolarization effects. Fewer loops are required to depolarize a broadband source than a narrower band source. Furthermore, if one wishes to depolarize without controlling the input polarization, theoretically one can never obtain a completely depolarized light. One can only hope that one will tend towards complete depolarization if one adds a great number of stages. The only advantage of this device is that it can be made with a low cost fused non-polarization maintaining couplers, but it requires several couplers, splices and a lot of optical fibers for all the different loops and thus is very bulky and lossy.
A more efficient recirculating loop is presented in U.S. Pat. No. 5,218,652 where control of the states of polarization is provided. Because the states of polarization are controlled, a single recirculating loop is required, reducing the amount of fiber and components. The principle is, however, the same as the recirculating loops discussed in U.S. Pat. No. 5,933,555. Part of the light is recombined with a delay and in a different state of polarization than the input light. In U.S. Pat. No. 5,218,652, the idea is to use a polarization preserving coupler to split the light and a birefringent fiber to preserve the polarization in the recirculating loop. It is stated that the best configuration for this device is to split the light with one part in the transmission and two parts in the loop and to recombine in the coupler at a 90xc2x0 angle, in essence rotating the polarization maintaining (PM) fiber by 90xc2x0 before splicing it to the second input of the coupler. In principle, one can achieve complete depolarization in this manner, if there is no optical loss in the coupler and the fiber loop, but in practice because the coupling ratio of the coupler depends on wavelength, the loop is lossy due to the length of the birefringent fiber and the precise control of the splice angle required in the loop. Because of such losses, the optical signal cannot infinitely loop back. However, because this configuration requires a single loop of polarization maintaining PM fiber, it is ultimately less lossy and bulky than the previously discussed design, but it still has the same kind of efficiency problem.
A way to address this problem is to use the loop as a delay line rather than a recirculating loop. This is present in Japanese Patent No. JP6051243 and U.S. Pat. No. 5,293,264. In both patents, a polarization beam splitter is used to separate the polarization in two parts and then recombine them at 90xc2x0 after looping the second part in a birefringent fiber which is reconnected at 90xc2x0 to the second input of the polarization splitter. Because the polarization splitter is used as a polarization combiner, the signal travels only once in the loop and is not recirculated. If the two parts, recombined at 90xc2x0, are of equal power, then the signal is completely depolarized. The depolarization is limited by the degree of polarization separation achievable by the polarization splitter. If it does not separate the polarizations perfectly, it can not recombine them perfectly, and thus the depolarization will not be perfect. Another restriction on this device is that the state of polarization at the input must be known and must then be aligned properly with the polarization beam splitter so as to split the signal in such a manner that the first part, being equal to the ratio of power split by the polarization beam splitter minus the loss in the polarization beam splitter, is equal to the second part, being the ratio of power split in the second output port minus the loss of the fiber loop minus twice the loss of the polarization beam splitter. The main problem with this device is the loss of the components and the proper handling of polarization maintaining fiber, which will directly affect the efficiency of the depolarizer.
Another depolarizer principle based on birefringent fiber is similar to the use of the polarization beam splitter. As explained in U.S. Pat. Nos. 5,457,756; 5,486,916 and 5,881,185, a birefringent fiber, spliced at 45xc2x0 with the linearly polarized laser, will split the signal into two parts of equal power. Because of the birefringence, a delay will accumulate between the parts along the fiber. When the delay is larger than the coherence length of the source, the source becomes depolarized and one can splice the birefringent fiber to an ordinary fiber. If one wants to become independent of the state of polarization, one splices two long lengths of polarization maintaining fiber at a 45xc2x0 angle. The first length creates two distinct incoherent polarization states, but because the input state of polarization is not controlled, these two incoherent states are not necessarily equal in power, which is required to produce a depolarized signal. At the 45xc2x0 angle splice between the two lengths of polarization maintaining fiber, the two incoherent polarizations are each split in two orthogonal sates of polarization of equal optical power. Each pair of states then propagates in the second length of polarization maintaining fiber and becomes incoherent after an appropriate length. If the two lengths of the polarization maintaining fiber are different, so as not to recreate coherence between the polarization states, the output of the lengths of fiber is composed of four incoherent polarization signals, in which two pairs of signals are of equal power and orthogonal polarization. The signal is then depolarized. Because the delay is obtained only by the birefringence of the fiber, long lengths of fiber are required and only broadband lasers (several nanometers bandwidth) can be depolarized with this method. Moreover, use of several hundreds of meters of birefringent fiber is lossy and bulky; 15 cm of delay lines in a fiber loop is approximately equivalent to 300 meters of delay in a polarization maintaining fiber.
The present invention addresses the problem of efficiency of a recirculating loop depolarizer, the problem of loss and polarization control of the polarization maintaining delay line depolarizer and the coherent length limitation of the birefringent fiber depolarizer as well as making the depolarizer less bulky and more practical to use.
An object of this invention is to provide an all-fiber depolarizer to depolarize light in an optical fiber by splitting the light into two orthogonal polarizations of approximately equal power and delaying one of the polarizations in an optical fiber loop before it is recombined with the other polarization state.
Another object of this invention is to use a polarization splitting coupler to separate the two polarizations and to recombine them thereafter.
A further object of this invention is to control the state of polarization of the signal between a birefringent fiber and a polarization splitter or between two polarization splitters by controlling the length, geometry and angle alignment of the fiber or of the polarization splitters.
A still further object of this invention is to use a standard non-polarization maintaining fiber in the fiber loop. This simplifies the loop and provides low loss.
Yet another object of this invention is to realize a depolarizer independent of the input state of polarization by concatenating two polarization splitters with fiber loops and by adjusting the angle of polarization coming out of the first polarization splitter to be at 45xc2x0 to the polarization axis of the second polarization splitter. The two fiber loops preferably have different lengths so as not to recreate coherent polarization from one length to another.
Other objects and advantages of the invention will become apparent from the following description thereof.
This invention has two main embodiments. The first is a single stage depolarizer device that depolarizes a signal of linear or circular polarization coming from an optical fiber. This depolarizer is composed of a 2xc3x972 polarization beam splitter where one of the input ports is looped back into the second input port to combine the polarization. The fiber loop is made of standard non-polarization fiber and its length is greater, preferably by several times (ex. 3-5 times) the coherence length of the light source to be depolarized. The second embodiment is a concatenation of two such assemblies to make the depolarizer independent of input polarization state, the first assembly being used to create two signals of incoherent orthogonal state of polarization and the second one to depolarize each of the signals.
In the first embodiment, the present invention uses a polarization beam splitter to create, to separate and to combine orthogonal polarizations. The polarization of the signal entering the polarization beam splitter must either be circular or linear, but at an angle of essentially 45xc2x0 from the polarization axis of the polarization beam splitter. This causes the signal to be split into two orthogonal, equally powered, optical signals, one in each output port of the polarization beam splitter. The 45xc2x0 angle can be controlled at the input of the polarization beam splitter with a polarization controller as is well-known in the art or, if the signal input is a polarization maintaining fiber, by splicing this fiber with the input fiber of the polarization beam splitter at 45xc2x0 angle difference between the birefringence axis of the signal fiber and the polarization axis of the beam splitter. If the input fiber of the polarization beam splitter is not a polarization-maintaining fiber, the splice must be in the component or in a place where the distance between the splice and the internal polarization splitting component is small and the optical fiber is in a straight position. The splice can be further away from the component if the fiber is properly managed or aligned so that the polarization is properly aligned at the input of the polarization maintaining coupler.
The polarization beam splitter may have polarization maintaining PM fibers, but this is not essential for the depolarizer to work. It was surprisingly found that contrary to Japanese Patent No. JP6051243, the loop fiber does not have to be a PM fiber. When the polarization signal is looped back at the second input of the polarization beam splitter, the light is then again split into two polarizations. However, only the state of polarization that is identical to the polarization at the input of the fiber loop will be transferred to the transmission port and that polarization will be orthogonal to the initial polarization that was transmitted from the first input of the polarization splitter to the output transmission port. Using a polarization-maintaining fiber would insure, when properly aligned, that all the power in the loop is thus transferred. However, if a non-polarization maintaining fiber is used, only some power will be transmitted. The rest will be coupled back in the loop, but at 90xc2x0 polarization rotation with reference to the first time transfer through the loop. The process repeats itself at every loop. However, because of this rotation between the first and second time in the loop, more than 75% of the power is always coupled back in the transmission fiber after just two loops, thus making the recoupling of the recirculation much more efficient than with a non-polarization dependent power splitter. Furthermore, the recombined power is always in the right polarization angle to reduce the degree of polarization, property which is set by the polarization beam splitter. Finally, by using the standard non-polarization maintaining single mode fiber (SM fiber), the loss in the loop, even if there is a splice, can be made very small (less than 0.02 dB). When the polarization beam splitter is an all-fiber based device, such as the ones described in applicant""s Canadian Patent Application No. 2,354,903 and Canadian Patent Application filed concurrently herewith on even date, the losses due to the polarization beam combiner are also very small, thus making the loop very efficient. To compensate for this small loss, one could adjust slightly the input angle of 45xc2x0 to a value that would put a bit more power in the loop (ex. 46xc2x0-47xc2x0 angle) so as to achieve a perfect balance between the two incoherent output polarizations.
The efficiency of this device will be limited in bandwidth by the bandwidth of operation of the polarization beam splitter. If a fused coupler is used (such as described in Canadian Patent Application No. 2,354,903 which is incorporated herein by reference) which has a bandwidth of approximately 8 nm and where the polarization cross talk is smaller than 15 dB, the depolarizer will be efficient over that wavelength range, and will be able to reduce the degree of polarization to better than 3%. If a Mach-Zehnder based polarization beam splitter is used (such as described in Canadian Patent Application filed by the applicant concurrently herewith on even date and incorporated herein by reference), then the bandwidth can be more than 100 nm.
In the second embodiment, two such depolarizers are concatenated to achieve a similar effect as placing two lengths of birefringent fibers is series, spliced at a 45xc2x0 angle, as described above. The effect achieved is the same, namely such a concatenation is independent of the input signal polarization state, with the difference, however, that this invention will depolarize narrow band lasers because the loops can easily be made longer than the coherence lengths of the lasers as opposed to the impractical birefringent fiber lengths required to do the same. This embodiment of the invention therefore provides a much more universal depolarizer. Because of this universal nature, there are some differences with the single-loop depolarizer. First, this two-stage depolarizer can be entirely built with non-polarization maintaining (SM) fiber. The input fiber of the device can be any fiber, including a non-polarization maintaining SM fiber and the fiber in the loop, as was explained above can also be non-polarization maintaining SM fiber. The state of polarization does not need to be known at the input. This simplifies the fabrication of the polarization beam splitter because no alignment is needed between the input fiber and the polarization axes of the polarization splitter. The polarization axes of the two first incoherent polarizations are defined after the first loop by the polarization axes of the polarization beam splitter. The second stage polarization axes are also defined by the polarization axes of the second polarization beam splitter. When these two pairs of polarization axes are aligned at 45xc2x0 angle, by controlling the fiber length and geometry and by properly rotating the polarization beam splitters one in respect to the other, the two first incoherent polarization states coming out of the first stage will be each split into two equally-powered incoherent polarization states, thus making the output depolarized. As with the first embodiment, the depolarizer is very efficient when low-loss fiber based polarization beam splitters are used.
In essence, therefore, the all-fiber depolarizer of the present invention comprises
In essence, therefore, the all-fiber depolarizer of the present invention comprises
(a) a polarization beam splitter having two input fibers and two output fibers and a beam splitting region in-between having a polarizing axis;
(b) control means for controllably injecting polarized light from a light source with a given coherence length into one of the input fibers of the polarization beam splitter so that the polarization of the signal entering the beam splitter is circular or linear at a substantially 45xc2x0 angle from the polarizing axis of the beam splitter;
(c) a loop formed between the second input fiber and one of the output fibers of the beam splitter, said loop being made of a standard non-birefringent fiber and having a length greater than the coherence length of the light source;
(d) the second output fiber receiving and further transmitting incoherent depolarized light produced by the interaction of the polarization beam splitter and the loop.
The polarization beam splitter is preferably a 2xc3x972 beam splitter adapted to divide light entering through one of its input fibers into two linearly polarized beams that are orthogonal and are defined by the polarizing axis of the beam splitter.
The polarization beam splitter may include as the beam splitting region a fused fiber coupler or it may be a broadband polarization splitter having, for example, an all-fiber Mach-Zehnder structure.
The control means may comprise a birefringent polarization maintaining fiber controllably spliced to one of the input fibers or in lieu thereof may comprise a primary polarization beam splitter having two input fibers and two output fibers, in which one input fiber forms another loop with one output fiber, said loop having a length greater than the coherence length of the light source. This loop preferably has a different length than the loop used with the polarization beam splitter that is concatenated to and follows this primary beam splitter. This arrangement produces a two-stage universal depolarizer.
If fused couplers are used in the beam splitters, the two-stage depolarizer will by narrowband. The alignment of 45xc2x0 between the two stages is done by properly aligning the plane of the two couplers which define the polarization axes. If Mach-Zehnder structures are used, then only the two birefringent parts of the Mach-Zehnder must be aligned at 45xc2x0 of each other.
These details will become clearer in the preferred embodiments described below with reference to the appended drawings.